The use of rotary valves is well known in the industry. For example, the document “A Practical Guide to Rotary Valves” (April 2008—Ian Blackmore—Rota Val Ltd and Professor Mike Bradley—The Wolfson Centre, University of Greenwich—http://www.shapa.co.uk/pdf/rotary_valves.pdf) gives extensive details of rotary valves. In particular, rotary valves are used for conveying hot particle during the operations downstream of olefin polymerisation reactors, for example between the polymerisation section and the extrusion section.
U.S. Pat. No. 5,765,728 discloses a system which conveys chopped PET (chips/flakes) at ambient temperature from a supply duct to a methanolysis recovery reactor. The system includes a rotary vane valve which is operated under conditions preventing the condensation of vapors of process chemicals coming from the methanolysis reactor.
U.S. Pat. No. 1,524,270 discloses a system comprising a sealing spider (rotating at a constant rate) for conveying coal to a furnace wherein the quantity of coal fed is controlled (by using an upstream dam or breaker) in order to maintain a continuous flow of coal throughout the system.
One of the recurrent problems encountered during the use of the rotary valves relates to gas leakages occurring between the rotor blades and the casing; the space between the rotor blades and the casing will be referred hereinafter as the clearance. Said clearance and the consequent gas leakages are depicted in FIG. 1 for illustrative purpose.
In order to reduce the gas leakages, the clearance between the rotor and the casing has to be as small as possible. However, too small a clearance can cause problems at start-up due to the rapid thermal expansion of the rotor relative to the casing when initially contacted with hot powder. The thermal expansion of the rotor relative to the casing causes the rotor tips to make contact with the casing, resulting in subsequent damage to the rotary valve.
Several solutions have been proposed and tried, e.g. external heat tracing and/or lagging of the equipment. Unfortunately, said expensive solutions have proven not to be satisfactory, especially for large rotary valves. Consequently, these rotary valves used for high capacity conveying of hot powder require a sufficient clearance which brings back the above mentioned problem of gas leakages.